Field of the Invention
The present invention relates to a system and method for preventing and monitoring leakage of water from a tank liner.
Description of the Related Art
Nuclear fuel in use (or fresh nuclear fuel) and spent nuclear fuel (or used nuclear fuel) of research reactors and spent nuclear fuel of commercial reactors are stored in a storage tank to cool the nuclear fuel and to shield radioactive rays from escaping to the outside. To this end, a reinforced concrete structure is installed outside the storage tank to maintain rigidity thereof. A tank liner is installed on a wall and a floor of the storage tank. The tank liner is formed by welding metal plates, and is attached to reinforced concrete so that it serves to ensure a water level of the storage tank and to prevent damage to the reinforced concrete structure.
At present, the storage tank for storing the nuclear fuel is designed to install a leaking water collecting system between the tank liner and the reinforced concrete in consideration of potential damage to the tank liner to make it possible to collect water leaking from the storage tank. The leaking water collecting system is installed along a weld zone of the tank liner, and is designed such that leaking water generated by damage to the weld zone is collected to a leaking water storage tank. However, the leaking water collecting system includes the following technical problems.
First, the leaking water collecting system can ascertain whether or not the leakage of water occurs, but not where the leakage of water occurs. Known methods of ascertaining where the leakage of water occurs include a macrographic test for the inside of the storage tank, a test using installation of a dedicated device (for example, a vacuum box device), and so on. A test for all regions of the tank liner is very difficult to perform due to restriction of accessibility, applicability, and so on.
Second, a part of the water in the nuclear fuel storage tank during an operation period is always evaporated in the air. Therefore, it is difficult to distinguish a reduction in the water of the storage tank caused by slight damage to the tank liner from a reduction in the water of the storage tank caused by evaporation. This leakage of water leads to expansion of a damaged region of the tank liner, and can result in an accident of excessive leakage of water.
Third, when an excessive leakage of water occurs, the water of the storage tank leaks between the leaking water collecting system and the reinforced concrete to bring about loss of a nuclear fuel cooling function, damage to the reinforced concrete structure, and environmental discharge of radioactive substances.
In the related art, as illustrated in FIG. 1, the tank liner 2 on the top of the floor of the storage tank is formed by mutually welding several metal plates. Since a weld zone 2-1 is most vulnerable, there is a high possibility of the leakage of water occurring, and a leaking water collecting channel 5 is installed under the weld zone 2-1. Leaking water collected in the leaking water collecting channel 5 can be delivered to a leaking water reservoir 7 along a connecting pipe 6.
Since the leaking water collecting channel 5 is buried in the floor when a reinforced concrete layer 1 (hereinafter referred to as “concrete floor”) acting as an outer layer of the tank liner 2 is constructed, a bottom of the tank liner 2 and opposite upper ends 5-1 of the leaking water collecting channel 5 cannot be mutually welded when the tank liner 2 is installed on an inner surface of the concrete floor 1.
Thus, when the water of the storage tank, which leaks from the weld zone 2-1 of the tank liner 2, exceeds a flow rate at which the water flows into the leaking water reservoir 7, the water overflows the opposite upper ends 5-1 of the leaking water collecting channel 5 and soaks into the concrete.
In this case, there is no method of preventing the leakage of water and, as described above, it is very difficult to ascertain whether and where the leakage of water occurs. Moreover, there is problem that can be led to the environmental discharge of the radioactive substances.
That is, the leaking water collecting system developed so far is installed on the wall and bottom of the weld zone of the tank liner. Since the leaking water collecting system installed on the wall is installed before the reinforced concrete is poured in view of a working process, it can be coupled with the wall of the tank liner in an airtight structure. Since the leaking water collecting system installed on the bottom is buried in the reinforced concrete first and then the bottom of the tank liner covers an upper portion of the leaking water collecting system, the leaking water collecting system cannot be coupled with the bottom of the tank liner in an airtight structure.
Therefore, various test devices such as a macrographic test using a camera, a test using a vacuum box, and so on is developed for the purpose of a test for the nuclear fuel tank liner during an operation. However, these test devices have many restrictions of time and space in the event of testing the nuclear fuel tank liner, and are accompanied with many difficulties in ascertaining a leak. Further, when such a leak is discovered, rapid sealing work should be performed to prevent an additional leakage of water.
Accordingly, there is a need to develop means for effectively preventing the leakage of water from the tank liner and means for reliably finding out where the leakage of water occurs.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.